1. Field of the Invention
The present invention relates to improved extractions which provide better removal of extraneous materials or by-products produced while preparing aromatic ether bisimides and thus increasing the yield of the desired bisimide product. The invention also relates to extractions of by-products from continuous processes of preparing aromatic ether bisimides.
2. Background Art
Aromatic ether bisimides are useful as chemical intermediates in the production of a number of important compounds. For example, 2,2-bis[4-(N-methylphthalimide-4-oxyphenyl]propane (hereinafter "bisphenol A-bisimide" or "BPA-BI") is used in the preparation of polyetherimides which are commercially important thermoplastics. In certain processes aromatic ether bisimides are prepared from an alkali metal salt of a hydroxyaromatic compound which undergoes a nucleophilic displacement reaction with a substituted phthalimide. See, for example, U.S. Pat. No. 4,520,204. The displacement reaction is usually effected in a relatively non-polar organic diluent such as benzene, toluene, xylene, chlorobenzene, tetrahydrofuran, octane, acetonitrile or the like in the presence of a phase transfer catalyst, typically a tetraalkylammonium salt. See, for example, U.S. Pat. Nos. 4,257,953; 4,273,712; and 4,247,464. A frequently used diluent for this reaction is toluene.
The reaction mixture containing the prepared aromatic ether imide typically contains extraneous material or by-products, such as unreacted substituted phthalimide, phase transfer catalyst and various inorganic salts formed in the displacement reaction. Such by-products, unless removed, tend to compromise the properties of the end products which are subsequently prepared from the bisimides.
As indicated above, bisimides can be used to prepare polyetherimides. Specifically, BPA-BI is hydrolyzed to a tetraacid derivative and dehydrated to form BPA-dianhydride. BPA-dianhydride ("BPA-DA") is reacted with organic diamines (for example metaphenylene diamine) to make the desired polyetherimide. Synthesis of a suitable bisimide and its conversion to a dianhydride is more specifically disclosed and claimed in U.S. Pat. No. 3,879,428.
The thermal stability and other properties of polyetherimide resins are very much dependent on the purity of the BPA-DA used in their synthesis. It has been found that the amount of impurities in BPA-DA derived from BPA-BI often is greater than desirable. Such impurities typically include bisphenol-A, 4-nitrophthalimide and 4-bisphenol-A-monoimide. These by-products in the past were separated by extraction with aqueous and/or dilute aqueous alkali. See U.S. Pat. No. 4,520,204 to Evans. For example, extractions with dilute (1%) alkali hydroxide solutions heated to about 70.degree.-80.degree. C. have been satisfactorily employed in certain instances. See U.S. Pat. No. 4,247,464 to Relles et al. In fact, aqueous solutions of 1% by weight alkali hydroxide have been frequently relied upon to extract by-products from the precursors to numerous commercial polyetherimides. Processes employing 1% alkali metal hydroxide extraction solutions typically comprise multiple extraction steps in order to completely remove impurities from the reaction mixture, especially to completely remove the monoimide impurities which due to the fact that it has a solubility similar to the bisimide product is not easily removed from the organic phase. These multiple step processes, however, can be time consuming, create a large volume of aqueous waste and incur high disposal costs. The waste is usually incinerated and the energy consumption necessary to drive off the aqueous portion of the waste is high. Moreover, hydrolysis of bisimide occurs at each extraction, thereby reducing the yield of bisimide. Physical losses of bisimide are also incurred at each extraction/separation step.
The costs and disposal concerns of the current methods of extracting aromatic ether bisimide by-products has thus far made it difficult to efficiently prepare these bisimides. A continuous process would be desirable over a "batch" process for several reasons, one of which is efficiency that can be gained when preparing large volumes of bisimide. Typically, continuous processes also provide for a more consistent process and product.
Accordingly, there remains a need and desire for improved processes-for removing by-products and impurities created during the production of aromatic ether bisimides. Faster removal of impurities, the reduction of aqueous volume extract, and the alleviation of disposal problems, concentration and incineration costs incurred by current procedures are also desirable. These issues are addressed by the present invention.
It would also be preferable to reduce the hydrolysis of the desired bisimide and bisimide yield loss which occurs with the current extraction processes, as well as reduce the physical losses of bisimide that are incurred with each multiple/extraction step. Both of these, too, are objects of the present invention.
Moreover, an extraction process which goes towards meeting these goals can be used to improve a continuous process of preparing aromatic ether bisimides, with the improvement thereof also a goal of this invention.